1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image on a recording material, more particularly to an image forming apparatus such as a photocopier, a printer, a facsimile machine or a multi-functional apparatus including some of these functions.
2. Description of the Related Art
Heretofore, as a fixing system to fix an unfixed toner image onto a recording material, owing to safety and satisfactory fixing property, a thermal fixing system is generally used in which the unfixed toner image is heated, melted and fixed to the recording material.
Especially, owing to a satisfactory thermal efficiency and ease of miniaturizing, a thermal roller system is broadly used in which the unfixed toner image on the recording material is heated, pressurized and thermally fixed in a fixing region where a heat roller (roll) is brought into contact under pressure with a pressure roller.
In a fixing apparatus of the thermal roller system, there are used a fixing roller including a heater therein, and the pressure roller brought into contact under pressure with this fixing roller to face the fixing roller, and the recording material is introduced into a fixing nip portion between this pair of rollers to pass through the portion. Accordingly, the unfixed toner image formed and borne on the surface of the recording material is fixed onto the surface of the recording material by heat and pressure.
In recent years, from viewpoints of quick starting and energy saving, a fixing apparatus of a film (belt) heating system has been put to practical use.
In the fixing apparatus of the film heating system, a heat-resisting film (hereinafter referred to as the fixing film) is sandwiched between a ceramic heater as a heating member and the pressure roller as a pressurizing member to form the fixing nip portion. Moreover, the recording material on which the unfixed toner image is formed and borne is introduced between the fixing film of the fixing nip portion and the pressure roller, and sandwiched and conveyed together with the fixing film. Accordingly, while applying heat of the ceramic heater via the fixing film, the unfixed toner image is fixed onto the surface of the recording material by a pressurizing force of the fixing nip portion.
In the fixing apparatus of the thermal roller system or the film heating system described above, there is known a problem that a temperature of a non-sheet-passing portion rises during continuous passing of the recording material having a width which is smaller than that of the recording material having the maximum sheet passing width.
When the recording material having the maximum size is passed to fix the image, the surface of the heat roller has an approximately uniform temperature distribution over the whole length of the fixing region. However, in a case where the recording material having a small size is continuously passed to fix the image, the temperature of the surface of a non-sheet-passing region of the heat roller excessively rises. This is because when the recording material having the small size is continuously passed, in the non-sheet-passing region where any recording material does not pass, any heat is not taken by the recording material and the heat is partially accumulated.
To solve the problem, as disclosed in Japanese Patent Application Laid-Open Nos. S60-136779, H05-181382 and 2003-076209, there is proposed a constitution in which a temperature sensor is disposed to detect the temperature of a region of the heat roller as the non-sheet-passing portion during the fixing onto the small-sized recording material. In a case where the temperature detected by this temperature sensor rises at a predetermined temperature, the corresponding region of the heat roller is cooled by a fan.
In a case where the width of the recording material set by a user is larger than that of an actual recording material, however, even when the temperature detected by the temperature sensor is normal owing to a cooling effect of the fan, the temperature of a part of the region which must be a sheet passing portion of the heat roller might excessively rise. On the other hand, in a case where the width of the recording material set by the user is smaller than that of the actual recording material, a fixing defect might be generated in an end portion of the recording material in a width direction owing to the cooling effect of the fan.